As related arts, audio coding techniques for compressing audio signals (sound source of voice, music, etc.) are being developed. For example, as the audio coding techniques, the Advanced Audio Coding (AAC) system, the High Efficiency-Advanced Audio Coding (HE-AAC) system, and so forth exist. The AAC system and the HE-AAC system are one of Moving Picture Experts Group (MPEG)-2/4 Audio standards of ISO/IEC and are widely used for broadcast purposes such as digital broadcasting for example.
In the broadcast purpose, audio signals are transmitted under restrictions of a limited transmission bandwidth. Therefore, in the case of coding audio signals at a low bit rate, the band in which the coding is carried out is selected because it is difficult to code the audio signals in the whole frequency band. In general, in the AAC system, a bit rate equal to or lower than about 64 kbps can be regarded as a low bit rate and a bit rate equal to or higher than about 128 kbps can be regarded as a high bit rate. For example, Japanese Laid-open Patent Publication No. 2007-193043 discloses a technique in which coding is carried out with dropping of audio signals with power lower than given power so that the amount of bits for the coding may fall within a given bit rate.